Running, when integrated properly, can either complement resistance training or lead to overtraining and compromised results. “Leg Day” refers to a structured workout focused on stimulating the lower body muscles—glutes, quadriceps, hamstrings, and calves—with resistance to promote strength, power, and size. Whether running helps or hurts this goal depends on the type of running, its timing, and the athlete’s primary training objective. Integrating these two forms of exercise requires a strategic approach to training volume and recovery to maximize benefits while minimizing interference.
How Running Supports Leg Performance and Recovery
Low-intensity running, often termed active recovery, supports a leg-day routine. Gentle movement promotes increased blood flow to fatigued leg muscles following a heavy lifting session. This enhanced circulation aids in flushing out metabolic byproducts that accumulate during intense resistance work, potentially reducing delayed onset muscle soreness (DOMS).
This low-impact activity also delivers essential nutrients and oxygen to muscle tissues necessary for repair and rebuilding. Running is an effective way to build strength endurance—the ability to generate force over an extended period. While it does not build maximal strength like heavy lifting, it trains muscle fibers for repetitive, sub-maximal contractions that support overall leg function and stamina.
The Conflict Between Endurance and Strength Training
The primary challenge in combining running and resistance training, known as concurrent training, is the physiological “interference effect.” This effect describes how endurance training can compromise gains in muscle hypertrophy and strength sought during leg day. This conflict centers on two competing signaling pathways within muscle cells.
Resistance training activates the mammalian target of rapamycin (mTOR) pathway, the primary driver of muscle protein synthesis and growth. Conversely, high-volume endurance exercise, like long-distance running, activates the adenosine monophosphate-activated protein kinase (AMPK) pathway. AMPK activation signals the cell to conserve energy, which can transiently suppress the mTOR pathway, blunting the anabolic response needed for maximal strength and size gains.
High-volume running also places significant metabolic and mechanical demands on the muscle groups targeted by leg day. This leads to residual fatigue and glycogen depletion, limiting the ability to lift heavy weights with the intensity needed for strength adaptations. High-volume endurance work also shifts muscle characteristics toward a more fatigue-resistant, Type I dominance, which is counterproductive to the explosive force production goals of strength athletes.
Strategic Timing and Intensity for Optimal Results
The negative effects of concurrent training depend largely on the proximity and intensity of the two sessions. To prioritize strength and hypertrophy, a separation of at least six to eight hours between a running session and a heavy leg day is recommended, with 24 hours being more effective. If both must occur on the same day, perform the resistance training session first to ensure muscles are not fatigued, which is necessary for maximal force production.
The type of running matters. Low-intensity steady-state (LSD) running is generally benign and can be used as active recovery on off-days or after a light resistance session. High-intensity running, such as sprints or interval training, should be treated like another hard leg workout and scheduled on a separate day from heavy lifting. Running before a heavy leg day is generally a poor idea, as it depletes muscle glycogen stores and compromises the quality of the strength work.
Recovery Guidelines
For optimal adaptation, athletes focused on strength should allow 48 hours for muscle recovery following a heavy leg session before engaging in another high-intensity lower-body workout.